Is it just my imagination, or is Firefox a piece of crap and getting worse?

On computer "a" (LM 17.1) I cannot use the profile with all my preferences and bookmarks without it (Firefox) telling websites that it is running an older version, (which drives some sites crazy). If I use a completely new (empty) profile it delivers the correct version information and sites will run O.K., but bookmarks, etc. are gone.
Somewhere in those profile files lurk the few bits responsible for the misinformation, search as I may, I cannot find them.
Does anyone out there have any information about where those offending bits may be hiding so I can edit them out?

On computer "b", (also LM 17.1), every time I open it I see a page that says "Adblock has been installed" (https://getadblock.com/installed/?u=8tfnbjyj88807344), even though my preferences are set to: "when Firefox starts: show a blank page"
Can anyone help me eliminate this idiotic behaviour?

Do you have an ad blocker installed? Check in Add-Ons section. You may also have an Refer Agent spoofer add-on which is reporting older versions, or even different browser versions. Check all your Add-ons.

I do have "ad block", but it never acted in this manner before. Here's what I do have:

Perhaps these addons are fighting each other, but, I haven't changed them recently. Perhaps I should post another question about how to know which addons will conflict, render others superfluous, redundant, etc.
Thanks.

I have been living with this for quite a while. (machine "a"). I had upgraded Firefox, however, (not Linux Mint) when the problem arose. That is why I think there is something embedded within one of those files that is saying "version old.old" and if it could be found, and edited to read "version new.new" instead, the problem would be solved.

and see what version you are reporting to sites. For more interesting pages, also open

about:about

You do have a lot of add-ons. To check if those are causing it, turn them all off, then access same sites claiming you have old browser. If it continues, then you know it's not the add-ons. Do that from the Help section in Menu.

....P.erhaps I should post another question about how to know which addons will conflict, render others superfluous, redundant, etc.
Thanks.

...what i can tell you is that uBlock can perfectly replace 3 addons out of them, namely Adblock+Disable WebRTC+NoCoin.
Make sure to read some tutorial to get yourself familiarized with such first, it shouldn't take more than a few minutes...
Disconnect Basic can also be avoided, as it's filter lists are already included in Firefox (configure Edit - Preferences - Tracking Protection).

All in all, save yourself some extra memory, unnecessary cpu cycles and also possible conflicts...

Thanks THX-1138,
I read about ublock, and installed it on " a", (the machine I am now on) (which has a different list of addons than "b"), which I will do next.
One other question: Does ublock also eliminate the need for "Ad Block For You Tube" ?

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One other question: Does ublock also eliminate the need for "Ad Block For You Tube"
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...it accepts the exact same filter lists as Adblock, thereby all that someone would need, would be to add such a list for Youtube.
Eg. have a look here (search for 'Youtube', or say 'Coin'...or whatever annoys you for that matter, and click Subscribe on the desired ones...): https://filterlists.com/

However i can't really remember the last time i stumbled into such...and i haven't added specifically anything for YT myself.
In uBlock's settings, i've simply enabled all of Built-in, Ads, Privacy & Malware domains' lists...hence i assume those do the trick 'as is'?
Didn't went the extra mile of enabling 'Annoyances' as well...
But still, if such doesn't work for you somehow, just subscribe to some Youtube-blocking related lists from filterlists.com above...